Shak, it's true that thicker material absorbs to lower frequencies, this is explained in Ethan's FAQ under Optimizing the Air Gap - however, normal house insulation has a much lower density than the Rigid commercial fiberglas - Owens Corning 703 has 3 Pounds per Cubic Foot (abbreviated PCF), their 705 series has 6 PCF, but normal house insulation has a density of less than 1 PCF. The "quiet" stuff may be as much as 1 PCF, the normal is usually around .5 to .8 PCF.
It's this density that affects how MUCH absorption you get - it's the distance from boundaries (walls) that determines how LOW frequencies will be absorbed.
The reason for this is the way absorption works - the sound wave causes air molecules to move back and forth, except where there's a wall they can't move at all. When the sound is moving, anything it has to pass through causes the sound to have to try and move the fibers (interstices) of the material. Doing this causes the sound energy to be converted to heat energy, and since we can't hear heat, there is less sound. The more of these "interstices" (fibers) there are in a given volume of the absorbent, the more sound is absorbed (up to a point) - this is why the denser, Rigid, type of fiberglas does better at absorption.
So, basically, thick is better than thin, heavy is better than light, and more air space behind (or just thicker material) lowers the effective absorption frequency.
Ethan's FAQ covers most of this, so (if you don't get put in jail for stealing copy paper) keep it handy and read it several times... Steve
